Methods and apparatus for recovering gypsum and magnesium hydroxide

ABSTRACT

The present invention provides methods and apparatus for treating flue gas containing sulfur dioxide using a scrubber, and more particularly relates to recovering gypsum and magnesium hydroxide products from the scrubber blowdown. The gypsum and magnesium hydroxide products are created using two separate precipitation reactions. Gypsum is crystallized when magnesium sulfate reacts with calcium chloride. Magnesium hydroxide is precipitated when magnesium chloride from the gypsum crystallization process reacts with calcium hydroxide. The process produces a high quality gypsum with a controllable pH and particle size distribution, as well as high quality magnesium hydroxide.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/673,033 filed Apr. 20, 2005, entitled: “METHODSAND APPARATUS FOR RECOVERING GYPSUM AND MAGNESIUM HYDROXIDE PRODUCTS”.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for treating fluegas containing sulfur dioxide using a scrubber, and more particularlyrelates to recovering gypsum and magnesium hydroxide products from thescrubber blowdown.

BACKGROUND INFORMATION

Coal and oil fired power plants produce flue gas that contains sulfurdioxide. Limestone (calcium carbonate) and lime (calcium hydroxide) arethe traditional sources of alkalinity for scrubbing the flue gas fromcoal fired power plants. The purity of gypsum produced from alimestone-based scrubbing system varies between about 85 to 95 percent,with 90 percent being typical. The gypsum purity from other lime-basedscrubbing systems ranges from about 94 to 98 percent. The pH of gypsumproduced from a limestone-based scrubbing system is usually betweenabout 6.0 and 8.0, which is a desirable range for producing wallboard.However, the pH of gypsum produced from lime-based scrubbing systems canexceed 8.0, making the gypsum unsuitable for wallboard productionwithout further processing. The particle size distribution of the gypsumproduced from limestone-based scrubbing systems is usually a narrowrange of about 10 to 70 microns, whereas the gypsum produced fromlime-based scrubbing systems is a broad range of about 10 to 200microns. A narrow range of particle size distribution is desirable forthe production of wallboard. Lime-based scrubbing systems typically havea lower initial installation cost and a higher sulfur dioxide removalefficiency. However, the gypsum produced from a lime-based scrubbingprocess is generally not suitable for use without further processingsuch as pH adjustment, treatment with chemical modifiers, or grindingand/or screening.

In lieu of a limestone or lime-based scrubbing system, flue gascontaining sulfur dioxide may be treated by passing it through ascrubbing unit that utilizes a magnesium-containing scrubbing agent,e.g., a magnesium hydroxide slurry. The magnesium hydroxide reacts withsulfur dioxide to produce magnesium sulfite and magnesium bisulfite. Themagnesium hydroxide slurry also reacts with hydrochloric acid in theflue gas, producing magnesium chloride, although there is very littlehydrochloric acid in the flue gas compared to sulfur dioxide. Thefollowing chemical reactions occur in the scrubbing unit:SO₂+Mg(OH)₂→MgSO₃+H₂O  (1)H₂O+SO₂+MgSO₃→Mg(HSO₃)₂  (2)Mg(HSO₃)₂+Mg(OH)₂→2MgSO₃  (3)2HCl+Mg(OH)₂→MgCl₂→2H₂O  (4)

The blowdown from the scrubbing unit contains magnesium sulfite andmagnesium bisulfite. Several processes have been developed to convertthis blowdown into valuable products that can be collected for sale orfurther use, namely, gypsum and magnesium hydroxide. Gypsum may be usedto make wallboard and as a soil conditioner on large tracts of land insuburban areas as well as agricultural regions. Magnesium hydroxide is achemical reagent grade material that may be used for pH neutralizationin water treatment, and SO₃ removal and boiler slag prevention in coalfired power plants. The purity of magnesium hydroxide generally rangesfrom about 85 to 95 percent, and most magnesium hydroxide that is soldhas a purity ranging from about 88 to 92 percent.

One method for treating flue gas containing sulfur dioxide is marketedby Dravo Lime Company as the ThioClear® process. A wet scrubber with amagnesium hydroxide scrubbing agent is utilized to remove sulfur dioxidefrom the flue gas. The blowdown from the scrubber passes through anoxidation unit where magnesium sulfite and magnesium bisulfite areconverted to magnesium sulfate. Bleed from the oxidation unit is reactedwith a slaked lime slurry to crystallize gypsum and magnesium hydroxideparticles in a regeneration tank. Thus, the process converts magnesiumsulfate blowdown to gypsum and magnesium hydroxide using calciumhydroxide in a single chemical reaction.

U.S. Pat. No. 4,874,591 to Jeney discloses a process for thepurification of waste gas containing hydrochloric acid and sulfurdioxide. The waste gas is contacted with a magnesium-containingscrubbing agent, e.g., a magnesium hydroxide suspension, to generatereaction products. A calcium-containing reagent, e.g., calcium chloride,is introduced to the reaction products to precipitate gypsum andgenerate a chloride-containing liquid. The liquid undergoespyrohydrolysis to produce a stream of magnesium hydroxide, which isrecirculated to the scrubber, and a stream of hydrochloric acid, whichis recovered for use in other applications.

Japanese Patent No. JP1222524A to Morita discloses a treatment processfor waste water containing magnesium sulfate. The wastewater is mixedwith calcium chloride to form a gypsum product that is separated outusing a filter press. Calcium hydroxide is added to the separatedfiltrate to form magnesium hydroxide in a reaction tank. The reactionproduct is separated into magnesium hydroxide and a filtrate by a filterpress, and the magnesium hydroxide is used as the adsorbent for wetdesulfurization. Part of the filtrate is sent to a water tank containingcalcium chloride and part is sent to a reverse osmosis unit. Watercontaining calcium chloride from the reverse osmosis unit is recycledinto the water tank. Magnesium hydroxide from the reverse osmosis unitis used in a slurry tank containing aqueous magnesium hydroxide slurry.

Because gypsum and magnesium hydroxide represent valuable products,there exists a need for new systems that are capable of efficientlyconverting scrubber blowdown into these substances. While the prior artprovides methods for generating these substances using amagnesium-containing scrubbing agent, new methods could improve theefficiency of existing systems, the quality of the gypsum and magnesiumhydroxide produced, and the ability to control pH and particle sizedistribution.

SUMMARY OF THE INVENTION

The present invention in a preferred embodiment provides methods andapparatus for treating flue gas containing sulfur dioxide andhydrochloric acid using a scrubber, and more particularly relates torecovering gypsum and magnesium hydroxide products from the scrubberblowdown. The gypsum and magnesium hydroxide products may be createdusing two separate precipitation reactions. Gypsum is crystallized whenmagnesium sulfate reacts with calcium chloride. Magnesium hydroxide isprecipitated when magnesium chloride from the gypsum crystallizationprocess reacts with calcium hydroxide. Because of the separateprecipitation reactions and other features discussed below, the processproduces a high purity gypsum with a controllable pH and particle sizedistribution, as well as high quality magnesium hydroxide.

An aspect of the present invention in a preferred embodiment is toprovide a method for recovering gypsum product and magnesium hydroxideproduct from flue gas containing sulfur dioxide, the method comprising:treating the flue gas containing sulfur dioxide with a magnesiumhydroxide slurry to produce magnesium sulfite and magnesium bisulfite;oxidizing the magnesium sulfite and magnesium bisulfite by reaction withair and magnesium hydroxide; producing magnesium sulfate blowdown fromthe oxidized magnesium sulfite and magnesium bisulfite; reacting themagnesium sulfate blowdown with calcium chloride to produce a gypsumslurry; removing gypsum fines from the gypsum slurry; washing the gypsumslurry after the gypsum fines are removed to produce gypsum product, amagnesium chloride filtrate, and a residual stream; reacting themagnesium chloride filtrate with calcium hydroxide to produce amagnesium hydroxide slurry; removing grit and unreacted calciumhydroxide from the magnesium hydroxide slurry to form a treatedmagnesium hydroxide slurry; and purifying the treated magnesiumhydroxide slurry to produce a magnesium hydroxide product and a calciumchloride solution.

Another aspect of the present invention in a preferred embodiment is toprovide an apparatus for recovering gypsum product and magnesiumhydroxide product from flue gas containing sulfur dioxide, the apparatuscomprising: a scrubber for removing sulfur dioxide from the flue gas andproducing magnesium sulfite and magnesium bisulfite, wherein saidapparatus is structured for oxidizing the magnesium sulfite andmagnesium bisulfite by reaction with air and magnesium hydroxide toproduce magnesium sulfate blowdown; a gypsum crystallization unit forreacting the magnesium sulfate blowdown with calcium chloride to producegypsum slurry; a gypsum fines separation unit for removing gypsum finesfrom the gypsum slurry; a dewatering filter belt for washing the gypsumslurry after the gypsum fines are removed to produce gypsum product, amagnesium chloride filtrate, and a residual stream; a magnesiumhydroxide production vessel for reacting the magnesium chloride filtratewith calcium hydroxide to produce magnesium hydroxide slurry; a gritremoval unit for removing grit and unreacted calcium hydroxide from themagnesium hydroxide slurry to produce a treated magnesium hydroxideslurry; and a magnesium hydroxide purification unit for purifying thetreated magnesium hydroxide slurry to produce a magnesium hydroxideproduct and a calcium chloride solution.

An object of the present invention is to improve the efficiency ofexisting systems that treat flue gas using a magnesium-containingscrubbing agent.

Another object of the present invention is to produce high qualitygypsum and magnesium hydroxide products from the treatment of flue gascontaining sulfur dioxide.

A further object of the present invention is to generate gypsum andmagnesium hydroxide products using two separate precipitation reactions.

Another object of the present invention is to provide control of pH andparticle size distribution for gypsum product that is produced.

These and other objects of the present invention will become morereadily apparent from the following detailed description and appendedclaims.

FIGURE

The FIGURE is a process flow diagram in accordance with a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides methods and apparatus for treating fluegas containing sulfur dioxide and a small amount of hydrochloric acidusing a scrubber, and more particularly relates to recovering gypsum andmagnesium hydroxide products from the scrubber blowdown. The gypsum andmagnesium hydroxide products are created using two separateprecipitation reactions. Gypsum is crystallized when magnesium sulfatereacts with calcium chloride. Magnesium hydroxide is precipitated whenmagnesium chloride from the gypsum crystallization process reacts withcalcium hydroxide. Because of the separate precipitation reactions andother features discussed below, the process produces high quality gypsumwith a controllable pH and particle size distribution, as well as highquality magnesium hydroxide. The quality or purity of the gypsumproduced may range from about 95 to about 100 percent. In a preferredembodiment, the gypsum quality ranges from about 97 to about 100percent. The purity of the magnesium hydroxide produced may range fromabout 85 to 95 percent, and preferably ranges from about 90 to 95percent.

In one embodiment of the present invention, a scrubber may be utilizedto treat sulfur dioxide-containing flue gas using a magnesium-containingscrubbing agent. Suitable magnesium-containing scrubbing agents includemagnesium hydroxide slurry. The scrubber blowdown, which containsmagnesium sulfite and magnesium bisulfite, passes through an oxidationunit that converts sulfite to sulfate, generating magnesium sulfate inthe blowdown of the oxidation unit. The magnesium sulfate blowdowntravels through a purification unit where inert material is removed. Thepurification unit outputs a magnesium sulfate solution that reacts witha calcium chloride solution in a gypsum crystallization unit. Thecrystallization unit produces gypsum slurry which passes through aseparation unit where gypsum fines are separated and recycled to thecrystallization unit. The gypsum slurry then passes through a dewateringfilter belt and gypsum product is produced for collection. The magnesiumchloride filtrate moves to a magnesium hydroxide production vessel whereit reacts with calcium hydroxide to form a magnesium hydroxide slurry.The magnesium hydroxide slurry passes through a grit removal unit suchas a hydroclone, then through a purification unit for removal of calciumchloride, which is recirculated to the gypsum crystallization unit. Thepurified magnesium hydroxide is recirculated to the scrubber orcollected for use in other applications.

The FIGURE presents a process flow diagram in accordance with apreferred embodiment of the present invention. Flue gas 2 containingsulfur dioxide and hydrochloric acid is diverted to a scrubber 4 thatutilizes magnesium hydroxide slurry 6 as a scrubbing agent and water 8.Sulfur dioxide is removed from the flue gas according to the followingreactions:SO₂+Mg(OH)₂→MgSO₃+H₂O  (1)H₂O+SO₂+MgSO₃→Mg(HSO₃)₂  (2)Mg(HSO₃)₂+Mg(OH)₂→2MgSO₃  (3)2HCl+Mg(OH)₂→MgCl₂+2H₂O  (4)

Once the sulfur dioxide is removed, the scrubbed flue gas 10 exits thescrubber 4 along with some evaporated water. A solution containingmagnesium sulfite and magnesium bisulfite is generated by the scrubbingprocess. Although the concentration of total sulfites (magnesium sulfiteand magnesium bisulfite) in this solution may vary, the concentration inthe scrubber may range from about 15,000 to 21,000 mg/L. A blowdown 12of the magnesium sulfite and magnesium bisulfite solution (referred toherein as “magnesium sulfite/bisulfite blowdown”) exits the scrubber 4and proceeds to an oxidation unit 14.

The amount of magnesium hydroxide slurry 6 added to the scrubber 4 isadjusted to control the pH of the magnesium sulfite/bisulfite blowdown12. The pH of the blowdown 12 is preferably maintained at about 6.0 to6.2, although the present invention is not limited to any particular pH.The pH within the scrubber 4 preferably ranges from about 5.8 to 6.5,although this pH may vary as well. The volume of continuous blowdown 12may be controlled to maintain a constant sulfite concentration in thescrubber 4. In a preferred embodiment, the flow rate of the blowdown 12is proportional to the volume of magnesium hydroxide slurry 6 enteringthe scrubber 4.

The oxidation unit 14 inputs air 16 and magnesium hydroxide slurry 18 toconvert the magnesium sulfite/bisulfite blowdown 12 into magnesiumsulfate. The conversion proceeds according to the following reactions:MgSO₃+½O₂→MgSO₄  (5)Mg(HSO₃)₂+O₂→MgSO₄+H₂SO₄  (6)H₂SO₄+Mg(OH)₂→MgSO₄+2H₂O  (7)

Although the present invention is not limited to any particularconcentration of magnesium sulfate, the concentration in the oxidationunit 14 may range from about 150,000 to 400,000 mg/L.

Because the oxidation of magnesium sulfite/bisulfite produces sulfuricacid, magnesium hydroxide slurry 18 is fed to the oxidation unit 14 tocontrol the pH of magnesium sulfate produced. It is preferable if themagnesium hydroxide slurry 18 is fed to the oxidation unit 14 at a pHranging from about 5.0 to 5.5. It is also preferable if the pH in theoxidation unit 14 ranges from about 5.0 to 6.4. However, the presentinvention is not limited to any particular pH value.

The oxidation unit 14 produces a stream of magnesium sulfate 15 that maybe recirculated to the scrubber 4. Since the oxidation of magnesiumsulfite/bisulfite to magnesium sulfate is an exothermic reaction, theexcess heat can be extracted from this stream with a heat exchanger 17and used as a heat source for a calcium chloride concentration unit 78in a manner discussed hereinafter. The recirculated stream of magnesiumsulfate 15 increases the concentration of magnesium sulfate in thescrubber/oxidizer circuit, which reduces the size and cost of equipmentthat is ultimately required to produce gypsum.

In addition to the recirculated stream of magnesium sulfate 15, aportion of the magnesium sulfate is removed from the oxidation unit 14as magnesium sulfate blowdown 20. The amount of blowdown 20 may beadjusted to maintain the concentration of magnesium sulfate belowsaturation levels. The flow of magnesium sulfate blowdown 20 may becontrolled by measuring the conductivity of the recirculated stream ofmagnesium sulfate 15 since the magnesium sulfate is the majorconstituent of the recirculated stream.

To achieve the desired pH of the final gypsum product (about 5.0 to 8.0for wallboard production, for example), the pH of the magnesium sulfateblowdown 20 may be controlled. This pH can be controlled by adjustingthe amount of magnesium hydroxide slurry 18 that enters the oxidationunit 14. In addition, the pH can be controlled by introducing additionalmagnesium hydroxide 22 to the process stream if necessary.

The magnesium sulfate blowdown 20 is delivered to a magnesium sulfatepurification unit 24 such as a thickener, filter, or centrifuge toremove any inert material, calcium sulfate, fly ash, or unburned carbonthat enters the process with the flue gas, makeup water, or lime slurry.Inert material may include iron oxide, aluminum oxide, or silica oxide.Removal of this material improves the purity of the final gypsumproduct. In lieu of the purification unit 24, the magnesium sulfateblowdown 20 may be sent to a surge tank (not shown) or directly to thegypsum crystallization unit 30. A surge tank may be incorporated betweenthe oxidation unit 14 and the gypsum crystallization unit 30 to maintainthe process flow at a controlled, constant rate. This could reduce thesize of the downstream equipment if the amount of magnesium sulfateblowdown 20 varied due to station loading.

The solids 26 that are separated out in the purification unit 24 may besent to a landfill for disposal. A magnesium sulfate solution 28 exitsthe purification unit 24 and enters a gypsum crystallization unit 30,where the magnesium sulfate is mixed with calcium chloride 32. Thechemical reaction proceeds as follows:MgSO₄+CaCl₂→CaSO₄↓+MgCl₂  (8)

The gypsum crystallization unit 30 is designed to produce crystals withthe desired particle size distribution. In a reaction typecrystallization unit, particles of varying sizes can be made. Theparticles are usually separated using an upward solution flow. Gravitypushes the larger particles to the bottom, while the gypsum product andsmaller particles move to the top. The larger particles are pulled fromthe bottom and the smaller particles are either left in the gypsumcrystallization unit 30 to continue to grow or are used as “seed”material. The rate of the upward solution flow determines the finalmaximum particle size. Although the present invention is not limited toany particular particle size distribution, the gypsum crystallizationunit 30 may be designed to produce a product with about 95 percent ofthe particles in a narrow range, for example, about 20–100 microns,which is desirable for the production of wallboard. However, the gypsumcrystallization unit 30 may also be designed to achieve a particle sizedistribution that is even narrower than 20–100 microns if desired.Alternatively, a broad range, for example, about 10–290 microns, may beproduced.

In one embodiment, an excess of calcium chloride 32 may be mixed withthe magnesium sulfate solution 28 in the gypsum crystallization unit 30so that all of the sulfate, or a majority of the sulfate, reacts and isremoved.

After crystallization, gypsum slurry 34 leaves the gypsumcrystallization unit 30 and enters a gypsum fines separation unit 36.The gypsum fines 38 are recirculated to the gypsum crystallization unit30. The concentration of gypsum solids in the gypsum crystallizationunit 30 is preferably maintained at a minimum of about 30 percent solidsand a maximum of about 40 percent solids.

The gypsum slurry 40 exits the gypsum fines separation unit 36 andproceeds to do dewatering filter belt 42. The gypsum slurry is washedone or more times with water or some other suitable liquid, which leavesbehind a high quality gypsum product 48 that may be collected for saleor further use. In a preferred embodiment, the gypsum slurry is washedtwice. The first wash 44 generates a magnesium chloride filtrate 50 thatcontains residual water from the washing process and magnesium chloridefrom the crystallization of gypsum. The second wash 46 generates aresidual stream 52 that can be sent for waste water treatment anddisposal, or more preferably, can be recirculated back to the scrubber4. Since the residual stream 52 will contain some magnesium chloride,returning it to the scrubber will preserve the magnesium and chloridewithin the process. The residual stream 52 can also go to a magnesiumhydroxide production vessel 58 if desired.

The magnesium chloride filtrate 50 is sent to a magnesium hydroxideproduction vessel 58 where it is mixed with calcium hydroxide 54.Depending on the design of the gypsum crystallization unit 30, a portionof the magnesium chloride filtrate 50 can be recirculated back andblended with the magnesium sulfate solution 28 that enters the gypsumcrystallization unit 30.

The following reaction occurs within the magnesium hydroxide productionvessel 58:MgCl₂+Ca(OH)₂→Mg(OH)₂+CaCl₂  (9)The calcium hydroxide 54 that is fed to the production vessel 58 may besupplied in the form of a lime slurry (Ca(OH)₂). It is preferable forthe slurry to contain a minimum of about 0.5 percent magnesium in theform of magnesium oxide or magnesium hydroxide to compensate for lossesof magnesium throughout the system. Magnesium oxide may be added to thelime or it may be naturally occurring in the limestone from which thelime is made. Dolomitic lime which contains up to 30 percent magnesiumoxide may be used. If lime containing no magnesium hydroxide is used,then magnesium hydroxide will need to be added to the process. It isdesirable for the calcium hydroxide 54 lime slurry to have a totalsolids content ranging from about 15 to 20%, although the presentinvention is not limited to any particular total solids content.

The production vessel 58 generates a magnesium hydroxide slurry 56 thatalso contains calcium chloride. Although the present invention is notlimited to any particular pH, the pH in the production vessel 58 may bemaintained in the range of about 8.8 to 9.9 to ensure that all or amajority of the calcium hydroxide is reacted, and little or no calciumhydroxide remains in the stream leaving the production vessel 58.However, a small amount of unreacted magnesium chloride in the magnesiumhydroxide slurry 56 may be acceptable. The pH may vary depending onunknown factors such as contaminants in the lime slurry, reaction vesseldesign, pH measuring point, and flow rates.

In a preferred embodiment, the magnesium hydroxide production 58 islarge enough to provide a four to six hour retention time to ensureadequate time to react the calcium hydroxide 54 with the magnesiumchloride 50 and to prevent downstream scaling.

The magnesium hydroxide slurry 56 may be pumped through a grit removalunit 59, for example, a hydroclone cluster, to remove any grit orunreacted lime particles. Although the description contained hereinprimarily refers to the use of hydroclones, it is to be understood thatthe present invention also contemplates the use of other, similar gritremoval devices, e.g., screens, provided that the separation unit doesnot add any additional water to the process. The hydroclone underflow 60is returned to the magnesium hydroxide production vessel 58 in the formof filtrate 66. Alternatively, or periodically, the hydroclone underflow60 may be collected, filtered, or screened 62 to remove largerparticles, which would be collected for disposal in a landfill 64. Thefiltrate 66 from the screening step, which contains calcium chloride insolution, may be returned to the magnesium hydroxide production vessel58 to minimize losses of chloride from the system. The screens 62 may bewashed with water 67 and a mild hydrochloric acid solution 68 to resistclogging and provide a source of chloride ions to further compensate forchloride losses throughout the process. Alternatively, the filtrate 66could be sent for waste water treatment and disposal if there issufficient chlorine in the coal to compensate for chloride lossesthroughout the system.

The hydroclone overflow, referred to herein as the treated magnesiumhydroxide slurry 70, may be diverted to a magnesium hydroxidepurification unit 72. The purification unit 72 separates the calciumchloride solution 74 from the treated magnesium hydroxide slurry 70,leaving behind a high quality magnesium hydroxide product 76 or slurry.A portion of the magnesium hydroxide product 76 may be recirculated tothe scrubber 4 for use as the magnesium hydroxide slurry 6.Additionally, a portion of the magnesium hydroxide product 76 may berecirculated to the oxidation unit 14 or used for pH control 22. Anyexcess magnesium hydroxide product 76 may be collected for sale orfurther use. Although the present invention is not limited to anyparticular concentration, the concentration of the magnesium hydroxideproduct 76 is preferably maintained relatively constant and if possiblewithin the range of 50 to 60 percent magnesium hydroxide, to minimizethe amount of water introduced to the scrubber with the slurry and tomake it a more marketable product. If the concentration of the magnesiumhydroxide product 76 is maintained constant, the magnesiumsulfite/bisulfite blowdown 12 may be controlled as a ratio of themagnesium hydroxide slurry 6, making it unnecessary to frequentlymonitor the sulfite concentration in the scrubber.

The calcium chloride solution 74 produced from the purification unit 72may receive further processing. The volume of this calcium chloridesolution 74 may be more than double and possibly three times the volumeof calcium chloride that was fed to the crystallization unit 30. Thecalcium chloride solution 74 may proceed to a concentration unit 78,where excess water 79 is removed. In a preferred embodiment, a vacuumpump 80 may be used to facilitate evaporation. Excess heat produced fromthe reactions in the oxidation unit 14 could be recovered with a heatexchanger 17 to assist with the concentration (evaporation of water) ofthe calcium chloride, further reducing energy costs. In addition, heatrecovered from a heat exchanger 84 used to condense the water vapor 86could be returned to the concentration unit 78 to further assist withthe concentration of the calcium chloride solution. In one embodiment, asource of heat 77 could be supplied to the calcium chlorideconcentration unit 78 in the event recovery of heat from the process isimpractical. This source of heat 77 could be in the form of steam fromthe boiler.

The calcium chloride stream 88 from the concentration unit 78 may bepurified using hydrochloric acid 90 for pH adjustment, and the purifiedcalcium chloride solution 89 may be recirculated back to the gypsumcrystallization unit 30 as stream 32. The purification of the calciumchloride stream 88 should improve the quality of the final gypsumproduct 48. To achieve such purification, hydrochloric acid 90 may beinjected into the calcium chloride stream 88 to lower the pH of thissolution to about 6.0–7.0, thereby removing any remaining calcium ormagnesium hydroxide and resisting scaling in the calcium chlorideconcentration unit 78. However, the pH adjustment may vary, and thedesired pH of the final gypsum product 48 typically determines the pHcontrol setpoint. The hydrochloric acid stream 90 provides a source ofchloride to make up for losses in the system.

In a preferred embodiment, the concentration of calcium chloride in thepurified calcium chloride stream 89 may range from about 200,000 to400,000 mg/L. However, the present invention is not limited to anyparticular concentration of calcium chloride in this stream. The designof the gypsum crystallization unit 30 will determine the concentrationof the purified calcium chloride stream 89. If the process requiresremoval of some chlorides, it is preferable to have a higherconcentration of calcium chloride in this stream. The concentration ofthe stream can be easily controlled using the calcium chlorideconcentration unit 78.

If coal being burned in the boiler contains enough chlorine, there maybe an excess of calcium chloride solution 92 that could be sold asanother product of the process. This would also provide a means forremoving the chlorides from the process without putting them to waste.

Any calcium chloride remaining in the magnesium hydroxide slurry 76recirculated to the scrubber 4 or oxidation unit 14 will react to formeither calcium sulfite or calcium sulfate. The calcium sulfite will beoxidized to calcium sulfate in the oxidation unit 14. This calciumsulfate will be removed in the magnesium sulfate purification unit 24.The calcium sulfate formed in the oxidation unit 14 could cause scalingrequiring periodic cleaning of the unit.

An advantage of oxidizing the magnesium sulfite/bisulfite blowdown 12 tomagnesium sulfate and recirculating it 15 to the scrubber 4 is that itreduces the amount of magnesium sulfate blowdown 20, thereby reducingthe capital costs of the plant. Concentrating the magnesium sulfatereduces the size of capital equipment required down stream of theoxidation unit 14, making this process more attractive than other gypsumprocesses. For example, at an operating process temperature of about 125degrees Fahrenheit, magnesium sulfate is soluble up to a concentrationof over 500,000 ppm. The side stream blowdown could, theoretically, bereduced to 200 gpm or less. Oxidation could be performed in either thescrubber sump or the recirculation tank of the scrubber. However, theexcess oxygen in the gas stream may cause unknown problems.

In alternative embodiments of the present invention, the systemcomponents may be modified or eliminated. For example, the magnesiumpurification unit 24 or the second belt wash 46 may be eliminated, whichwould reduce capital costs but may also reduce the quality of the gypsumthat is produced. The oxidation unit 14 may also be eliminated, andoxidation of magnesium sulfite/bisulfite to magnesium sulfate may occurdirectly in the scrubber 4. To achieve such oxidation in the scrubber 4,air may be introduced along with the magnesium hydroxide slurry 6. Whilethe present invention focuses on the use of “units” (e.g., the oxidationunit, magnesium sulfate purification unit, gypsum crystallization unit,etc.), it is to be understood that the present invention is not limitedto any one unit for accomplishing the production of gypsum and magnesiumhydroxide.

EXAMPLES

Several experiments were conducted to demonstrate the formation ofgypsum using calcium chloride solution and magnesium sulfate, todetermine the purity and pH of the gypsum, and to test the results ofusing highly concentrated solutions of magnesium sulfate and calciumchloride. An additional experiment was conducted to demonstrate thatmagnesium chloride reacts with calcium hydroxide to form magnesiumhydroxide and calcium chloride.

Example 1

A 500 ml saturated solution of calcium chloride was prepared usingdemineralized water. A 3000 ml straight-sided beaker was used as thereaction vessel. The reaction vessel was filled to the 2000 ml mark withfines thickener overflow from a power station gypsum process. Thissolution contained approximately 45,000 mg/L of magnesium sulfate. Apaddle type mixer was used to agitate the contents of the reactionvessel at 100 RPM. The pH of the solution was 6.60.

The calcium chloride solution was introduced to the reaction vessel in10 ml increments. After adding 40 ml, fine particles began to form. Theagitator was shut off to observe the crystal formation. As the solutionslowed, the crystals began forming faster and getting larger.

Agitation started again at 40 RPM. The calcium chloride solution wasintroduced in 10 ml increments until 250 ml had been added. Thecrystallization continued to the point of being unable to see throughthe solution. The agitator was shut off, allowing the solids to settle.Once settled, the solids filled the beaker to the 625 ml mark on the3,000 ml beaker. The final pH of the solution was 6.04.

As the gypsum solids concentration increased, the reaction rate betweenthe magnesium sulfate and the calcium chloride appeared to increase andthe particles grew larger. This is consistent with crystallizationtheory.

The solids were filtered through a No. 41 Whatman filter paper usingvacuum filtration. The filtrate was clear. The beaker was rinsed withabout 400 ml of demineralized water in order to transfer all of thegypsum to the filter funnel. After dewatering the gypsum, it was rinsedtwice with 200 ml of demineralized water. The gypsum filtered easily.The solids were white and fluffy, and the particles appeared extremelysmall.

The solids were transferred to a pan for drying. Six grams of wet solidswere removed for a moisture and purity test on the Arizona Instrumentmoisture and purity balance. The remaining solids were dried at 45degrees Celsius for 2 hours. The Arizona Instrument test showed a freemoisture content of 45% and a purity of 101.8%. Most likely the smallsize of the particles allowed them to retain moisture, resulting in theelevated moisture content. The small particle size also makes it moredifficult to rinse the gypsum in the filter, thus leaving some calciumchloride salts behind in the gypsum. This salt will give a false highpurity as indicated by a purity greater than 100%. If the gypsum wasadequately rinsed, it is believed that the purity would be close to, butnot exceed, 100%.

A pH test was performed on the dried solids. The solids were pulverizedand 10 grams were mixed with 100 ml of demineralized water for 15minutes in a 250 ml disposable beaker. The pH of the slurry after mixingwas 7.3.

Example 2

A 300,000 mg/L solution of magnesium sulfate was prepared, along with a300,000 mg/L solution of calcium chloride. 100 ml of the magnesiumsulfate solution was transferred to a 500 ml beaker and stirred at about100 RPM. The calcium chloride solution was introduced in 5 mlincrements. After adding the first 5 ml, particles began forming. Afteradding 25 ml, the slurry appeared thick and the crystals were clumping.After adding 40 ml of the calcium chloride solution, the slurry wasthick enough to form peaks. After adding 50 ml, the slurry was so thickit had to be mixed by hand. This slurry was filtered, leaving somemagnesium sulfate unreacted.

The slurry was filtered through a Whatman # 41 filter. The filtrate wasclear with a pH of 6.08. The solids were rinsed with about 700 ml ofdeionized water and dried at 40 degrees Celsius for three hours. Apurity test performed on an Arizona Instrument moisture balance gave apurity of 100.77%. The purity greater than 100% is a result of somemagnesium sulfate and calcium chloride remaining in the solids.

Example 3

A 150,000 mg/L solution of magnesium sulfate with a pH of 6.92 wasprepared, along with a 150,000 mg/L solution of calcium chloride with apH of 6.66. 100 ml of the magnesium sulfate solution was transferred toa 500 ml beaker and stirred at about 100 RPM. The calcium chloridesolution was introduced in 5 ml increments. After 10 ml of the calciumchloride solution were added, particles began forming. The thickness ofthe slurry increased as more calcium chloride was introduced, but theslurry continued to mix easily until the entire 100 ml was added. The pHof the slurry after adding the 100 ml of calcium chloride was 5.42.

The slurry was filtered through a Whatman # 41 filter, generating 110 mlof filtrate. The gypsum solids were rinsed with about 700 ml ofdeionized water. The solids appeared white. They were dried at 40degrees Celsius for two hours. A purity test performed on an ArizonaInstrument moisture balance showed a purity of 101.31%. A purity greaterthan 100% indicates that all of the magnesium chloride was not rinsedfrom the solids.

Example 4

A 200,000 mg/L solution of magnesium sulfate was prepared along with a150,000 mg/L solution of calcium chloride. A small amount of magnesiumhydroxide was introduced to the calcium chloride solution to increaseits pH to 7.43. 100 ml of the magnesium sulfate solution weretransferred to a 500 ml beaker and stirred at about 100 RPM. The calciumchloride solution was introduced in 5 ml increments. After adding 40 ml,the slurry became thick but could still be easily mixed. The calciumchloride solution was introduced in 5 ml increments until 125 ml hadbeen added. After adding 125 ml, the slurry was thick but could still beeasily mixed. The weight of the slurry produced was 234.2 grams. Thevolume was 210 ml.

The slurry was filtered through a # 41 Whatman filter. The pH of thefiltrate was 7.07. The volume of filtrate was 140 ml. The solids wererinsed with about 1000 ml of deionized water and dried for 2 hours at 40degrees Celsius.

Example 5

Saturated calcium chloride solution was added to 1000 mL of finesthickener overflow to produce a solution containing magnesium chloride.Excess calcium chloride was added to ensure the removal of magnesiumsulfate. The following reaction occurred:MgSO₄+CaCl₂→MgCl₂+CaSO_(4(s))  (10)The resulting solution contained magnesium chloride with some calciumchloride and calcium sulfate solids. This solution was filtered througha No. 41 Whatman filter paper, and the filtrate was collected.

1000 mL of the filtrate was transferred into a 2000 mL beaker and placedon a stir plate. A pH probe was inserted into the solution. A 17% totalsolids lime slurry solution was prepared using analytical grade calciumhydroxide and demineralized water, and the solids were allowed tosettle.

The initial pH of the magnesium chloride solution was 5.8. As thesolution was mixed, the clear portion of the calcium hydroxide solutionwas added in 20 mL increments. The pH of the solution increased slowly.At a pH of 8.5, a fine precipitate began to form. The addition of thecalcium hydroxide solution continued as before, and the pH continued toincrease. At a pH of approximately 9.85, the increase in pH ended. Theprecipitate continued to form as more calcium hydroxide solution wasadded. After using up the clear portion of the calcium hydroxidesolution the slurry was added to continue the reaction. As additionallime slurry was added, the pH of the solution increased slowly. Afteradding 900 mL of lime slurry, the solution was allowed to mix for 10minutes. The resultant as filtered through a No. 41 Whatman filter paper(25 micron). The solids were dried at 105 degrees Celsius for testing.The filtrate was clear, indicating that most of the solids formed werelarger than 25 microns in size. However, the solids appeared very fine,and several filter papers were required to filter them.

The precipitation reaction proceeded as expected with an equivalencepoint of 9.85 to 9.90 being established. Two additional tests wereperformed with identical results.

The present invention provides methods and apparatus for treating fluegas containing sulfur dioxide using a scrubber, and more particularlyrelates to recovering gypsum and magnesium hydroxide products from thescrubber blowdown. The gypsum and magnesium hydroxide products arecreated using two separate precipitation reactions. Gypsum iscrystallized when magnesium sulfate reacts with calcium chloride.Magnesium hydroxide is precipitated when magnesium chloride from thegypsum crystallization process reacts with calcium hydroxide. Theprocess produces a high quality gypsum with a controllable pH andparticle size distribution, as well as high quality magnesium hydroxide.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variation of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

1. A method for recovering gypsum product and magnesium hydroxideproduct from flue gas containing sulfur dioxide, the method comprising:treating the flue gas containing sulfur dioxide with a magnesiumhydroxide slurry to produce magnesium sulfite and magnesium bisulfite;oxidizing the magnesium sulfite and magnesium bisulfite by reaction withair; producing magnesium sulfate blowdown from the oxidized magnesiumsulfite and magnesium bisulfite; reacting the magnesium sulfate blowdownwith calcium chloride to produce a gypsum slurry; removing gypsum finesfrom the gypsum slurry; washing the gypsum slurry after the gypsum finesare removed to produce gypsum product, a magnesium chloride filtrate,and a residual stream; reacting the magnesium chloride filtrate withcalcium hydroxide to produce a magnesium hydroxide slurry; removing gritand unreacted calcium hydroxide from the magnesium hydroxide slurry toform a treated magnesium hydroxide slurry; and purifying the treatedmagnesium hydroxide slurry to produce a magnesium hydroxide product anda calcium chloride solution.
 2. The method of claim 1, wherein the fluegas containing sulfur dioxide is treated in a scrubber, the magnesiumsulfate blowdown reacts with calcium chloride in a gypsumcrystallization unit, the gypsum slurry is washed on a dewatering filterbelt, the magnesium chloride filtrate reacts with calcium hydroxide in amagnesium hydroxide production vessel, grit and unreacted calciumhydroxide are removed in a grit removal unit, and the treated magnesiumhydroxide slurry is purified in a purification unit.
 3. The method ofclaim 2, wherein the magnesium sulfite and magnesium bisulfite areoxidized in an oxidation unit using air.
 4. The method of claim 3,further comprising producing a stream of magnesium sulfate from theoxidation unit, wherein the stream of magnesium sulfate is recirculatedto the scrubber.
 5. The method of claim 3, further comprisingcontrolling pH of the magnesium sulfate blowdown with magnesiumhydroxide in the oxidation unit.
 6. The method of claim 2, wherein themagnesium sulfite and magnesium bisulfite are oxidized in the scrubberusing air.
 7. The method of claim 2, further comprising controlling pHof the magnesium sulfate blowdown by introducing additional magnesiumhydroxide to the magnesium sulfate blowdown.
 8. The method of claim 2,wherein the magnesium sulfate blowdown is adjusted to a pH ranging fromabout 6.0 to 8.0 prior to reacting with calcium chloride in the gypsumcrystallization unit.
 9. The method of claim 2, further comprisingremoving inert material from the magnesium sulfate blowdown in amagnesium sulfate purification unit prior to the blowdown reacting withcalcium chloride in the gypsum crystallization unit.
 10. The method ofclaim 9, wherein the magnesium sulfate purification unit utilizes amethod selected from the group consisting of thickening, filtering, andcentrifuge.
 11. The method of claim 2, wherein the magnesium sulfateblowdown passes through a surge tank prior to the blowdown reacting withcalcium chloride in the gypsum crystallization unit.
 12. The method ofclaim 2, wherein the magnesium sulfate blowdown reacts with an excess ofcalcium chloride in the gypsum crystallization unit.
 13. The method ofclaim 2, further comprising recirculating the gypsum fines to the gypsumcrystallization unit.
 14. The method of claim 2, wherein the residualstream from the dewatering filter belt is recirculated to the scrubber.15. The method of claim 2, wherein the calcium hydroxide is provided ina slurry that contains at least 0.5 percent magnesium.
 16. The method ofclaim 15, wherein the calcium hydroxide is provided as lime thatcontains magnesium oxide.
 17. The method of claim 15, wherein thecalcium hydroxide is provided as dolomitic lime.
 18. The method of claim2, wherein the grit removal unit comprises a hydroclone.
 19. The methodof claim 18, wherein underflow from the hydroclone is recirculated tothe magnesium hydroxide production vessel.
 20. The method of claim 2,further comprising concentrating the calcium chloride solution usingvacuum evaporation and heat.
 21. The method of claim 2, furthercomprising adjusting pH of the calcium chloride solution usinghydrochloric acid.
 22. The method of claim 2, further comprisingrecirculating the calcium chloride solution to the gypsumcrystallization unit.
 23. The method of claim 2, further comprisingcollecting the gypsum product for use or sale.
 24. The method of claim2, further comprising recirculating at least some of the magnesiumhydroxide product to the scrubber.
 25. The method of claim 2, furthercomprising recirculating at least some of the magnesium hydroxideproduct to the oxidation unit.
 26. The method of claim 2, furthercomprising collecting at least some of the magnesium hydroxide productfor use or sale.
 27. The method of claim 2, wherein quality of thegypsum product ranges from about 95 to about 100 percent.
 28. The methodof claim 2, wherein quality of the gypsum product ranges from about 97to about 100 percent.
 29. The method of claim 2, wherein pH of thegypsum product ranges from about 6.0 to 8.0.
 30. The method of claim 2,wherein particle size distribution of the gypsum product ranges fromabout 20 to 100 microns for about 95 percent of particles.
 31. Themethod of claim 2, wherein purity of the magnesium hydroxide productranges from about 85 to 95 percent.
 32. The method of claim 2, whereinpurity of the magnesium hydroxide product ranges from about 90 to 95percent.